The long-term goal of this project is the elucidation of the mechanisms of protective immunity against spotted fever group rickettsiae. A strong framework of knowledge of the mechanisms of protective immunity that clear disseminated endothelial infection established by intravenous inoculation of Rickettsia conorii into susceptible C3H mice has identified an important gap in our knowledge, namely of the initial infection events. This competing renewal application proposes to close that gap by experiments focused on intradermal inoculation, rickettsial interactions with dendritic cells, T lymphocyte priming in the draining lymph nodes, dendritic cell-NK cell cross talk, and immunomodulatory effects of Rhipicephalus sanguineus tick saliva. Preliminary results indicate that R. conorii activates dendritic cells, which influence the outcome of infection. The specific aims are 1) Determine the differences between the responses of dendritic cells of inbred mice genetically resistant or susceptible to Rickettsia conorii infection when the dendritic cells (DCs) are activated by the rickettsiae in vitro and in vivo and the effects of the presence of tick saliva in the rickettsial inoculum on the rickettsia-DC interaction and 2) Determine the role of activation of DCs by R. conorii on the pathogenesis of spotted fever rickettsiosis in vivo, including the effects on innate and adaptive immunity, and the effects of tick saliva in the rickettsial inoculum on the anti-rickettsial immune response. The experimental design takes advantage of well-established mouse models including one with complete innate resistance, one with dose-dependent mortality caused by R. conorii, and one employing R. australis that causes dose-dependent mortality in C57BL/6 mice enabling use of gene knockout approaches. In vitro studies, flow cytometry, real time PCR measurement of rickettsial load, immunohistochemistry, cytokine assays, ELISPOT assays, CTL assays, and other immunological approaches will be utilized in critically designed experiments to determine the effective generation of anti-rickettsial immunity.